Ruling is first step in sentencing of the comedian and actor, known as 'America's Dad', for sexual assault.

A judge declared actor and comedian Bill Cosby a "sexually violent predator" on Tuesday as a first step toward sentencing the 81-year-old comedian for drugging and sexually assaulting a woman over a decade ago.

The classification means that Cosby must undergo monthly counseling for the rest of his life and report quarterly to authorities. His name will appear on a sex-offender registry sent to neighbours, schools and victims.

Pennsylvania Judge Steven O'Neill made the decision as he prepared to sentence Cosby for violating Temple University women's basketball administrator Andrea Constand at his suburban Philadelphia estate in 2004.

The comic once known as America's Dad faced anywhere from probation to 10 years in prison after being convicted in April in the first celebrity trial after the #MeToo movement was popularised following a number of allegations against well-known individuals in October 2017.

Prosecutors asked for five to 10 years in prison. The defence requested house arrest, saying Cosby is too old and helpless to do time behind bars. He is legally blind.

Cosby's lawyers had fought the "sexually violent predator" designation, arguing that Pennsylvania's sex-offender law remains unconstitutional and that he is no threat to the public at his age.

But O'Neill said prosecutors had met their burden of proof by "clear and convincing" evidence.

When the ruling came down, a woman in courtroom shot her fist into the air and whispered, "Yessss!"

'I've been unable to heal'

Constand spoke briefly on the first day of the hearing on Monday, saying "all I am asking for is justice as the court sees fit".

In a statement submitted to the court and released on Tuesday, the 45-year-old said she has had to cope with years of anxiety and self-doubt that have left her "stuck in a holding pattern".

Constand said her training as a professional basketball player had led her to think she could handle anything, but "life as I knew it" ended on the night that Cosby knocked her out with pills and violated her.

Constand said she now has trouble trusting people.

"When the sexual assault happened, I was a young woman brimming with confidence and looking forward to a future bright with possibilities," she wrote in her five-page statement.

"Now, almost 15 years later, I'm a middle-aged woman who's been stuck in a holding pattern for most of her adult life, unable to heal fully or to move forward."

In the years since Constand first went to authorities in 2005, more than 60 women have accused Cosby of sexual misconduct, though none of those claims have led to criminal charges. Cosby's first trial in 2017 ended in a mistrial when jurors could not reach a unanimous verdict.

The judge, on Tuesday, ruled on Cosby's sex-offender status after a defence psychologist, Timothy Foley, testified that the chances of the comedian committing another sex offense are "extraordinarily low" because he is old, legally blind and needs help getting around.

On Monday, a psychologist for the state testified that Cosby appears to have a mental disorder that gives him an uncontrollable urge to assault women.

Cosby was smiling and joking with his spokesman and sheriff's deputies as he settled into the courtroom Tuesday. On the first day of the sentencing, the comic laughed at times as the psychologist on the stand for the state portrayed him as a sexual predator with signs of a mental disorder. Cameras were not allowed in the courtroom.

Cosby spokesman Andrew Wyatt said the former TV star planned to remain silent when given the opportunity to address the court. Cosby did not testify at either of his two trials.

The proceedings took place as women and others across the US protested against Supreme Court nominee Brett Kavanaugh, who faces allegations of sexual misconduct by two women. Kavanaugh has denied the allegations.

Tuesday's sentencing was a reckoning accusers and prosecutors said was decades in the making for the once-beloved entertainer known for his role as wise and understanding Dr Cliff Huxtable on the top-ranked, 1980s-era Cosby Show.

Cosby became the first black actor to star in a prime-time TV show, I Spy, in 1965. He remained a Hollywood A-lister for much of the next half-century.